There have previously been employed several methods for forming UBM or bumps on the silicon wafer. They include one which comprises performing a zinc substitution treatment to form a zinc film on an aluminum thin film electrode patterned on a wafer and performing electroless nickel plating thereon to form bumps; one which comprises performing a palladium treatment in place of the zinc substitution treatment and performing electroless nickel plating, thereby forming bumps; and one which comprises substituting a surface of an aluminum thin film electrodes directly with nickel and performing self-catalyzed electroless nickel plating to form bumps.
Any of these methods involves pretreatment before formation of UBM or bumps. This pretreatment generally comprises a degreasing treatment of the aluminum thin film electrodes and a treatment for removing an aluminum oxide film or metal impurities on the aluminum thin film electrodes. In this case, if an extremely thin film of aluminum oxide formed by immersion in nitric acid or the like is present, a subsequent plating step can be preformed directly without problem. However, if a strong aluminum oxide film remains on the surface after shaving or annealing, or if a specific crystal orientation plane exists on the aluminum surface, a plating film formed in subsequent steps will be poor in adhesion, or will be perforated. At worst, a plating film will not be formed on the surface. Such a strong aluminum oxide film is desired to be completely removed beforehand. Regarding the specific crystal orientation plane of the aluminum surface, the aluminum surface should be uniformly prepared.
In order to address the foregoing problem, there has been proposed a method for preparing the ground for plating by dry process without dissolving an aluminum oxide film (see Patent Document 1: JP-A 11-87392). However, this method still has room for improvement in terms of a complicated process, disadvantages in promptness and production costs. Moreover, the method has a problem that the electrical insulation of the remaining oxide film leads to an increased thermal resistance, resulting in deterioration of electrical characteristics.
It has been common practice to remove the strong aluminum oxide film by dipping in a strong alkaline or acid solution which dissolves not only an aluminum oxide film but also an aluminum or aluminum alloy base. This practice is applicable only if the base is sufficiently thick but is inapplicable if the aluminum or aluminum alloy is as thin as 0.5 or 1.0 m, in which case the etching margin is limited.
There have been proposed other methods, such as one which employs an organic solvent (see Patent Document 2: JP-A 2002-151537) and one which employs a mixture of several acids (see Patent Document 3: JP-A 5-65657, and Patent Document 4: JP-A 2002-514683).
These methods, however, involve difficulties in establishing adequate treatment conditions since the base material is inevitably etched excessively and where the base material is a thin film, the thin film may be disappeared or dissolved. Moreover, a conventional grinding or mechanical polishing cannot be adopted for the thin film, unlike the case of die casting. Hence, the oxide film formed by heat treatment in the fabricating process remains on the surface of the aluminum thin film, which worsens the problem.
In order to solve the foregoing problem, there has been proposed a remover which contains a salt or oxide of a metal capable of substituting aluminum, a solubilizing agent for the metal ions, an alkali and preferably a surfactant, with a pH value of 10 to 13.5 (see Patent Document 5: JP-A 2008-169446). This remover, when applied to an aluminum oxide film that has formed on aluminum or aluminum alloy, can rapidly remove the aluminum oxide film at a low temperature with minimum corrosion to the surface of aluminum or aluminum alloy.
That is to say, the reason of the severe corrosion of the base of aluminum or aluminum alloy when the conventional acid-based treating solution is applied to remove an aluminum oxide film is that there has been no effective method to deal with the difference between reactivity of aluminum oxide film with acid and reactivity of aluminum ground or aluminum alloy ground with acid.
JP-A 2008-169446 (Patent Document 5) discloses that an aluminum oxide film can be effectively removed by an alkaline remover comprising a salt or oxide of a metal capable of substituting aluminum and a solubilizing agent for the metal ions, as the result of investigation to dissolve and remove the aluminum oxide film while avoiding the high reactivity of a aluminum ground or aluminum alloy ground with acid.
FIG. 2 is a schematic sectional view showing the method in which the conventional alkaline remover removes an aluminum oxide film on the surface of aluminum or aluminum alloy. FIGS. 2(1) to 2(6) represent each step of removing an aluminum oxide film on the surface of aluminum or aluminum alloy, respectively. Incidentally, the reference numerals 1, 2, 3, and 4 in FIG. 2 denote aluminum or aluminum alloy having the (111) plane, aluminum or aluminum alloy having the (100) plane, an aluminum oxide film, and a metal derived from the additive metal capable of substituting aluminum, respectively.
The procedure starts with immersing aluminum or aluminum alloy having the aluminum oxide film 3 formed thereon in the conventional alkaline remover (containing zinc as the additive metal), as shown in FIG. 2(1), so that the aluminum oxide film 3 is removed, as shown in FIG. 2(2). As the result, the aluminum or aluminum alloy exposes itself. The aluminum or aluminum alloy 1 having the (111) plane, however, is subjected to a rapid substitutional deposition of the metal 4 on its surface, which is derived from the additive metal contained in the alkaline remover, as shown in FIG. 2(3).
The metal derived from the additive metal does not deposit substitutionally on the aluminum oxide film 3 because aluminum in the aluminum oxide film 3 has already ionized. Moreover, the aluminum or aluminum alloy 1 having the (111) plane is exempt from corrosion because it is protected by the deposited metal 4 formed on the exposed area. As this reaction proceeds, the deposited metal 4, which is derived from the additive metal capable of substituting aluminum, continues to deposit on the surface of aluminum or aluminum alloy 1 having the (111) plane which has exposed itself as the aluminum oxide film 3 is dissolved, as shown in FIG. 2(4). Eventually, the aluminum oxide film 3, which has existed on the surface of aluminum or aluminum alloy 1, is completely dissolved and removed. At the same time, the surface of aluminum or aluminum alloy is entirely covered with the deposited metal 4 which is derived from the additive metal capable of substituting aluminum, as shown in FIG. 2(5). The deposited metal 4 can be removed by acid cleaning, as shown in FIG. 2(6).
In other words, as shown in FIG. 2, the alkaline remover disclosed in JP-A 2008-169446 (Patent Document 5) does not corrode the aluminum ground or the aluminum alloy ground because it causes the deposited metal to cover immediately the aluminum base or the aluminum alloy base having the (111) plane which has been exposed by etching. Moreover, it continues to effectively remove the aluminum oxide film because its action to dissolve the aluminum oxide film is not impeded by the increasing concentration of aluminum hydroxide associated with dissolution of the aluminum base or the aluminum alloy base.
The alkaline remover offers another advantage of capability of accomplishing a treatment in a shorter time at a lower temperature than acid removers because it contains a large number of hydroxide ions (OH−) which can readily dissolve an aluminum oxide film.
Unfortunately, when the conventional alkaline remover is applied to the aluminum or aluminum alloy 2 having the (100) plane, substitution by the additive metal does not readily take place on its surface. In other words, only the dissolution of aluminum proceeds and substitution by the additive metal (zinc) shown in FIGS. 2(3) and 2(4) does not take place. Thus, the surface becomes smooth as shown in FIGS. 2(5) and 2(6). As the result, there is a problem that zinc substitution does not take place on the (100) plane in the subsequent process.
In the long run, the conventional alkaline remover brings about etching alone without substitution of aluminum by the additive metal on the specific crystal orientation plane (or the (100) plane). The result is that zinc substitution of aluminum does not take place on that crystal orientation plane in the subsequent processing, i.e., a lack of zinc substitution develops. The lack of zinc substitution causes the subsequent nickel plating to give defective nickel film, which is poor in adhesion and which is partly perforated. Such a defective nickel film is detrimental to electrical conductivity and appearance.
Listed below are prior art documents concerning the present invention.